Clinical pain encompasses nociceptive and neuropathic pain. Each type of pain is characterized by hypersensitivity at the site of damage and in adjacent normal tissue. While nociceptive pain usually is limited in duration and responds well to available opioid therapy, neuropathic pain can persist long after the initiating event has healed, as is evident, for example, in the “ghost pain” that often follows amputation. Chronic pain syndromes such as chronic neuropathic pain are triggered by any of a variety of insults, including surgery, compression injury or trauma, infectious agent, toxic drug, inflammatory disorder, or a metabolic disease such as diabetes or ischemia.
Neuropathic pain is a unique kind of chronic pain that is distinct from acute pain or inflammatory pain. Neuropathic pain, in contrast to most types of pain, persists in the absence of any detectable, on-going tissue injury process. It is common in patients that experience nerve injury in conditions such as diabetic neuropathy, post-herpetic neuralgia and chemotherapy-induced neuritis. A common feature is the occurrence of allodynia, which is defined as the perception of normally innocuous stimuli as being painful.
Unfortunately, chronic pain such as chronic neuropathic pain generally is resistant to available drug therapy. Furthermore, current therapies have serious side-effects such as cognitive changes, sedation, nausea and, in the case of narcotic drugs, addiction. Many patients suffering from neuropathic and other chronic pain are elderly or have medical conditions that limit their tolerance to the side-effects associated with available analgesic therapy. The inadequacy of current therapy in relieving neuropathic pain without producing intolerable side-effects often is manifest in the depression and suicidal tendency of chronic pain sufferers.
As alternatives to current analgesics, α2 adrenergic agonists, which are devoid of respiratory depressant effects and addictive potential are being developed. Such drugs are useful analgesic agents when administered spinally. However, undesirable pharmacological properties of α-adrenergic agonists, specifically sedation and hypotension, limit the utility of these drugs when administered orally or by other peripheral routes. Thus, there is a need for effective analgesic agents that can be administered by oral or other peripheral routes and that lack undesirable side-effects such as sedation and hypotension. The present invention satisfies this need and provides related advantages as well.
Also provided herein are new therapies for chronic pain sufferers, who, until now, have faced a lifetime of daily medication to control their pain. Unfortunately, available treatments for chronic neuropathic pain, such as tricyclic antidepressants, anti-seizure drugs and local anesthetic injections, only alleviate symptoms temporarily and to varying degrees. No available treatment reverses the sensitized pain state or cures pain such as neuropathic pain. Effective drugs that can be administered, for example, once or several times a month and that maintain analgesic activity for several weeks or months, are presently not available. Thus, there is a need for novel methods of providing long-term relief from chronic pain. The present invention satisfies this need and also provides related advantages.